Electrically heatable laminated window

ABSTRACT

A transparent, laminated window, particularly for use as a vehicle windshield, has improved deicing capability and high luminous transmission and near IR rejection, comprises an optical element having a low sheet resistivity of less than about 3 ohms per square, such element containing at least two transparent metal layers electrically conductively associated with bus bar(s) and separated by one or more dielectric layers.

CROSS REFERENCE TO RELATED APPLICATION

"Electrically Heatable Laminated Window," Floyd E. Woodward, U.S. Ser.No. 084,600, Filed: 8-11-87.

BACKGROUND OF THE INVENTION

This invention relates to transparent laminated windows and moreparticularly to such windows having improved defrosting capability.

With the advent of aerodynamic motor vehicle bodies having severelysloping front and rear windows to reduce drag and conserve fuel,automobile windows are increasing in surface area with each model year.This has disadvantages in that the inside of the motor vehicle becomesquite hot in summer while in winter it takes longer to defrost or deiceand defog quickly and uniformly using conventional motor vehiclegenerator voltages. As disclosed in U.S. Pat. No. 4,017,661, transparentlaminated windows which can be electrically defrosted are known.However, the amount of heat producible with motor vehicle generatedvoltages in such prior art windows is limited, which disadvantageouslyextends the time for clearing extended surface windshields. It would bedesirable to reduce the time for clearing laminated windows of ice andcondensation.

SUMMARY OF THE INVENTION

Now improvements have been developed which alleviate shortcomings of theprior art and facilitate removal of ice and condensation from laminatedwindows.

Accordingly, a principal object of this invention is to provide animproved electrically conductive motor vehicle windshield.

Another object is to provide a transparent, laminated window havingimproved resistive heating capability for removing ice and condensationfrom its surfaces.

A specific object is to provide an improved, electrically conductivewindshield containing an optical element capable of both hightransmittance (more than 70%) and low reflection (less than 16%) ofvisible radiation required for motor vehicle applications andsignificant rejection, primarily by reflection, of near infrared (IR)solar radiation, to minimize heat buildup in the vehicle compartment.

Other objects of this invention will in part be obvious and will in partappear from the following description and claims.

These and other objects are accomplished by electrically connectingfirst and second metal layers of an optically efficient multi-layermetal/metal oxide interference filter element to a source of electricenergy to decrease sheet resistivity and approximately double the powerdeliverable to the element (vis-a-vis one containing a single metallayer), thereby increasing electrically generated heat and reducingdefrost and/or demist time. The metal layers are spaced from each otherin the optical element by an amount which causes visible (i.e. luminous)light reflectance from each of the layers to destructively interfere soas to suppress undesirable luminous reflectance and enhance desirableluminous transmission of the window containing the optical element.

More specifically, a transparent, laminated window is provided havingimproved defrosting capability comprising: (A) an optical element havinga sheet resistivity of less than about 3 ohms per square containing: (i)at least two electrically conductive metal layers; (ii) one or moredielectric spacing layers separating such conductive layers; and (iii) atransparent substrate; and (B) a bus bar electrically conductivelyassociated with each conductive layer.

BRIEF DESCRIPTION OF THE DRAWING

In describing the invention, reference will be made to the accompanyingdrawing wherein:

FIG. 1 is a schematic, partial, cross-sectional view showing a typicalmulti-layer laminate window according to the invention; and

FIG. 2 is a view taken at 90° to FIG. 1

DETAILED DESCRIPTION OF THE INVENTION

The optical element component of the laminated window of the inventionis an interference filter of the Fabry-Perout type designed, principallythrough the appropriate selection of materials and their thicknesses tomaximize (i) transmission of visual or luminous and (ii) reflection ofheat-generating infrared portions (700-2125 nm) of the solar spectrum.Such element consists of multiple, sequentially deposited planar layersof angstroms-thick metal and dielectric coatings arranged in apredetermined sequence in face-adhering, contiguous contact with eachother, of the type generally disclosed in U.S. Pat. Nos. 3,682,528 and4,179,181.

The inventive optical element must contain at least two electricallyconductive, near IR reflecting metal layers which, when operativelypositioned in the element, result in a transmission of at least 70%visible light of normal incidence measured as specified in ANSI Z26.1,this being the minimum required in the U.S. automotive industry.Preferably visible light reflectance, normal from the surface of theelement is lsss than 16%. The metal layers of the optical element mustbe separated (i.e. vertically in the thickness direction) from eachother by one or more dielectric layers conventionally used ininterference filters so that reflection of visible light from the twoadjacent separated metal layers interferes destructively therebyenhancing visible transmission. Usable metals comprise silver, aluminum,chrome, nickel, brass, gold, stainless steel, copper, and alloys orcladdings of any of the foregoing. The preferred metal for each of thelayers is silver.

Each metal layer should be continuous and highly conductive to attainreduced thawing periods without deterioration in the visible lighttransmission and near infrared solar rejection properties of theelement. Reduced thawing time is achieved by using the second metallayer (though more than two may be present and each may be used) alongwith the first metal layer to conduct electricity for heating thewindow. The rapidity at which an applied voltage can defrost a window isdetermined by the sheet resistivity of the optical element. This is theresistance measured between buss bars extending across opposite sides ofa square element. Since vehicle windshields approximate a 2 by 1rectangle, the actual buss to buss resistance will be equal toapproximately one half of the sheet resistivity. The sheet resistivityof the optical element of the invention should not exceed 3 andpreferably be less than 2.5 ohms per square which, it is estimated,should provide a defrost time of less than two minutes at 0° F. Evenslight metal layer discontinuity drastically decreases electricalconductivity and luminous transmittance. To achieve desired continuity,the metal film should have a thickness of at least about 60 angstroms.Thinner films become unstable and tend to agglomerate and decrease inelectrical conductance and luminous transmittance with time. Thethickness, however, should not significantly exceed about 300 angstromsto avoid unacceptably low luminous transmittance.

The dielectric layer(s) of the optical element must be essentiallytransparent over the solar range (i.e. from 325 to 2125 nm) and at leastone must exist between each pair of electrically conductive metallayers. Preferably, a dielectric layer is positioned on each side of ametal layer. Exemplary usable dielectric materials include SiO, SiO₂,Ta₂ O₅, WO₃ O, In₂ O₃ . . . SnO₂, Al₂ O₃, In₂ O₃, MgF₂, ZnS, and Zn0₂ ;TiO₂ is preferred for each dielectric layer.

The substrate of the optical element comprises one or plural layers, oneof which directly supports the optical element in that a layer of thelatter rests in face to face contact on the substrate surface. Thesubstrate can be selected from a variety of materials with choicegenerally governed by the compatability between the optical element andthe substrate. Usable substrates should not be prone to stretch to avoidcracking the metal/dielectric layers and should be free of excessamounts of volatiles such as plasticizers, water vapor or absorbedgases. The dielectric layer of the optical element in direct contactwith the substrate should adhere well to the substrate surface.Generally the optical element of the invention adheres well to glass,ceramics and certain flexible plastics such as polyesters, castacrylics, polycarbonates, chlorinated plastics and epoxies.Polyurethanes and polyvinyl butyral as a substrate component in directsupportive contact with the optical element are too soft and extensible.Preferred substrates are sheets of transparent materials such as glassor non-extensible flexible plastic materials such as linear polyesters,e.g. polyethylene terephthalate which is commercially available asMylar®. In a preferred construction the optical element is sequentiallydeposited on a flexible sheet substrate of polyethylene terephthalate(PET), and then the substrate carrying the optical element isencapsulated within two layers of conventional plasticized polyvinylbutyral (PVB), one layer of which abuts the PET substrate and the otherabuts the top layer of the optical element. The multi-layered "sandwich"containing PVB as the outer layers is then conventionally laminatedbetween two rigid members such as glass panes, or alternatively may beused as a bilayer structure by laminating it to one such rigid memberintended to be the exterior side of a window.

Such a multi-layer structure 10 is shown in FIG. 1 and comprises glasslayers 12, 30, polyvinyl butyral layers 14, 28, PET layer 16, metaloxide layers 18, 22 and 26 and silver layers 20 and 24. FIG. 2 depictsthe window of FIG. 1 in the form of a vehicle windshield 32 having busbars 34, 36 electrically associated with silver layers 20 and 24.Besides glass, other rigid transparent sheets such as polycarbonate andacrylic are usable. Lamination must not significantly affect theelectrical conductivity of the optical element, nor reduce the visiblelight transmission or near IR reflectance by more than about 8%. Formotor vehicle window applications the flexible plastic substrate sheetof the sandwich referred to above should have a thickness of about 1 toabout 8 mils, the individual sheets of plasticized PVB should be about 5to about 60 mils thick and the rigid transparent member should be from60 to 500 mils thick.

As generally known in the art, varying the thickness and composition ofa dielectric layer spaced between two reflecting metal layers, will varythe optical transmittance/reflection properties of the optical elementconsiderably. More specifically, varying the thickness of the spacingdielectric layer varies the wave length associated with the reflectionsuppression (or transmission enhancement) band. In addition to choice ofmetal, its thickness also determines its reflectivity, the thinner thelayer, the less its reflectivity. For maximum reflection attenuation,the reflection of the two metal layers should be equal. As reflectivityof the two metal layers is reduced, the width of the reflectionsuppression band widens. Generally, the thickness of spacing dielectriclayer(s) should be between about 200 to about 1200 and preferablybetween 450 to 1000 angstroms to obtain the desired optical propertiesand film continuity necessary for a commercially acceptable product.Metal oxide dielectric layers less than about 200 or more than about1200 angstroms result in very low luminous transmittance.

Exterior dielectric layers in contact with the metal layer surfacesopposite to the metal surfaces contacting spacing dielectric layer(s)are preferably used to enhance anti-reflection performance. Exteriordielectric layers generally should have a higher refractive index thanglass or polyvinyl butyral, i.e. greater than 1.5 and preferably greaterthan 1.8. The thickness of such exterior or outside dielectric layer(s)is generally less than the spacing dielectric layer(s) and should beabout 100 to about 600 and preferably 160 to 500 angstroms.

Individual layers of the optical element are deposited by vacuum coatingtechniques well known in the art such as vacuum evaporation orsputtering. Usable methods include evaporation (resistance heated, laserheated, or electron-beam vaporization) and sputtering (diode ormagnetron) under normal or reactive conditions.

The laminated window of the invention can be used in any applicationrequiring a transparent resistance-heated window. The preferableenvironment is where fast deice or thaw intervals and good near IRrejection and luminous transmission of solar radiation is desired.Typical applications include automotive windshields and architecturalapplications such as commercial and residential buildings.

The invention is further described with reference to the followingexamples which are for illustration only and are not intended to implyany limitation or restriction on the invention.

All samples described in the Examples were prepared on 5 cm square 3 mmthick glass plate substrates. Prior to coating, i.e. deposition of theoptical element thereon, each glass plate was mechanically washed indetergent solution then rinsed and immersed in a 50/50 volume % solutionof concentrated sulfuric acid and 30% hydrogen peroxide. After removalthe plates were sequentially rinsed in distilled water and isopropranol,blown dry with nitrogen and placed in a vacuum chamber. After pumpingthe chamber to a base pressure less than 1×10⁻⁵ mbar, argon wasintroduced at a flow of 30 sccm to give a pressure of 3×10⁻³ m bar. Theglass substrates were rf etched (13.56 MHz, the resulting dc voltage was200 volts) for 10 minutes.

All vapor deposited coatings or layers were sequentially appliedseriatim, one on top of the other, by magnetron sputtering using aLeybold Heraeus sputter coater, the first layer being deposited on theglass plate surface. Titanium oxide and tungsten oxide were reactivelyprepared using round metallic targets which had a diameter of about 7.5cm and an area of about 44 square centimeters. Layer thicknesses weremonitored using an Inficon XTC crystal monitor.

Optical properties before and after lamination were determined with aPerkin Elmer 330 UV/VIS/NIR spectrophotometer. When reflectancemeasurements were made the optical element, i.e. the glass platecontaining the coating layers, was placed nearest the integratingsphere. Laminates were prepared with the outer vapor deposited layer inface to face contact with a PVB interlayer in the form of 30 mil thickSaflex® sheet. The sequential layers of a laminate were: optical element(i.e. coated glass substrate) /PVB/glass. In preparing a laminate, thecoated glass /PVB/glass stack was preheated at 150° C. for 15 min. and apressure of 40 psi applied for 5 min. while maintaining suchtemperature. With the pressure maintained the laminate was then allowedto cool to room temperature.

Sheet resistivities were measured with an Alessi Model A4P series fourpoint probe using a 3 volt power supply.

Abbreviations used in data in the Examples are defined as follows:

Tv, Rv and Av=respectively, visible transmission, reflectance andabsorption

Ts, Rs and As=respectively, solar transmission, reflectance andabsorption

SR=solar energy rejection calculated under standard summer conditionsspecified in ASHRAE Handbook, 1985, chapter 27--i.e. % SR=100%--%Ts--0.27× % As

RES=sheet resistivity in ohms per square

WO3=tungsten oxide

A=angstroms

Examples labeled "LAM" represent laminated samples while those withoutsuch designation were unlaminated--i.e. were only the glasssubstrate--multi-layer coating combination per se.

COMPARATIVE EXAMPLES C1-C3

These comparative Examples of an optical element containing a singlemetal layer are not according to the invention.

Optical element samples containing single silver layers of variousthicknesses and dielectric layers on either side of the silver layerwere prepared. Dielectric layer thicknesses were varied to achievemaximum visible transmission with each silver layer thickness.Deposition conditions and recipes were as follows:

    __________________________________________________________________________           Flow Rate                                                                              Total      Film  Deposition                                      Film                                                                              Argon                                                                             Oxygen                                                                             Press.                                                                             DC Power                                                                            Thickness                                                                           Rate                                         Ex.                                                                              Comp.                                                                             sccm                                                                              sccm mmbar                                                                              Watts A     A/sec.                                       __________________________________________________________________________    C1 TiO.sub.2                                                                         30  12.0 3.5  350   283   1.5                                             Ag  30  --   3    44    180   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   249   1.5                                          C2 TiO.sub.2                                                                         30  12.0 3.5  350   350   1.4                                             Ag  30  --   3    44    240   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   310   1.4                                          C3 TiO.sub.2                                                                         30  12.0 3.5  350   375   1.4                                             Ag  30  --   3    44    280   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   335   1.5                                          __________________________________________________________________________

Performance data obtained on each sample is given in the following TableI.

                                      TABLE 1                                     __________________________________________________________________________         Ag                                                                            Thickness                                                                Example                                                                            (A)   % Tv                                                                              % Rv                                                                              % Av                                                                              % Ts                                                                              % Rs                                                                              % As                                                                              % SR                                                                              RES                                    __________________________________________________________________________    C1   180   84.2                                                                              6.3 9.4 53.8                                                                              37.4                                                                              8.8 44  3.18                                   LAMCl      76.6                                                                              11.4                                                                              11.9                                                                              48.2                                                                              29.2                                                                              22.5                                                                              46                                         C2   240   85.2                                                                              7.2 7.6 52.0                                                                              40.9                                                                              7.0 46  2.24                                   LAMC2      72.1                                                                              17.2                                                                              10.7                                                                              42.7                                                                              36.5                                                                              20.8                                                                              52                                         C3   280   78.6                                                                              15.9                                                                              5.5 44.2                                                                              50.1                                                                              5.8 54  1.76                                   LAMC3      62.8                                                                              25.7                                                                              11.6                                                                              35.2                                                                              43.6                                                                              21.2                                                                              59                                         __________________________________________________________________________

From the above performance data; though Tv exceeds 70% at 180A silverlayer thickness in LAMC1, RES exceeds 3.0; at 280A silver thickness inLAMC3 RES is below 3 but % Tv is below 70; for LAMC2, though RES isbelow 3.0 and Tv exceeds 70% Rv is greater than 16%. For windshieldapplications 16% Rv is considered a maximum since in excess of this asafety hazard can be created in driving applications particularly duringnight driving where light within the vehicle can be dangerouslyreflected from the windshield into the eyes of the driver.

EXAMPLES 1 AND 2

These Examples containing dual metal layers are according to theinvention.

Optical element samples containing dual metal layers were prepared.

    __________________________________________________________________________           Flow Rate                                                                              Total      Film  Deposition                                      Film                                                                              Argon                                                                             Oxygen                                                                             Press.                                                                             DC Power                                                                            Thickness                                                                           Rate                                         Ex.                                                                              Comp.                                                                             sccm                                                                              sccm mmbar                                                                              Watts A     A/sec.                                       __________________________________________________________________________    1  TiO.sub.2                                                                         30  12.0 3.5  350   283   1.4                                             Ag  30  --   3    44    140   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   576   1.5                                             Ag  30  --   3    44    140   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   249   1.4                                          2  TiO.sub.2                                                                         30  12.0 3.5  350   283   1.4                                             Ag  30  --   3    44    180   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   576   1.5                                             Ag  30  --   3    44    180   20                                              WO.sub.3                                                                          30   1.0 3    18    60    1.0                                             TiO.sub.2                                                                         30  12.0 3.5  350   249   1.4                                          __________________________________________________________________________

The electrical and optical performance of each is given in the followingTable 2.

                                      TABLE 2                                     __________________________________________________________________________         Ag                                                                            Thickness                                                                Example                                                                            (A)   % Tv                                                                              % Rv                                                                              % Av                                                                              % Ts                                                                              % Rs                                                                              % As                                                                              % SR                                                                              RES                                    __________________________________________________________________________    1    140   77.6                                                                              3.5 18.9                                                                              44.2                                                                              41.2                                                                              14.6                                                                              52  2.10                                   LAM1       71.5                                                                              7.4 21.1                                                                              38.9                                                                              34.7                                                                              26.6                                                                              54                                         2    180   76.4                                                                              4.9 18.5                                                                              39.8                                                                              43.3                                                                              16.1                                                                              56  1.8                                    LAM2       72.9                                                                              7.4 19.8                                                                              37.2                                                                              36.0                                                                              26.9                                                                              56                                         __________________________________________________________________________

As evident in comparing the data for the laminated samples employing180A silver thickness in Example 2 in Table 2 and Example C1 in Table 1,with the inventive optical element of Example 2 RES is reduced by afactor of about 2 (i.e. from 3.2 to 1.8), visible normal reflectance isreduced from 11.4 7.4 and solar rejection is increased by 10% from 46%to over 50%--i.e. 56%.

While certain specific embodiments of the invention have been describedwith particularity herein, it will be recognized that variousmodifications thereof will occur to those skilled in the art. The scopeof the invention, therefore, is to be limited solely by the scope of thefollowing claims.

I claim:
 1. A transparent, laminated window having improved defrostingcapability comprising, in combination:(A) an optical element having asheet resistivity of less than about 3 ohms per square containing:(i) atleast two electrically conductive metal layers; (ii) at least onedielectric spacing layer between said conductive layers; and (iii) atransparent substrate; and (B) a bus bar electrically conductivelyassociated with each conductive layer.
 2. The window of claim 1 whereinthe conductive layers are silver having a thickness of from about 60 toabout 300 angstroms.
 3. The window of claim 1 wherein a dielectric layeris titanium dioxide having a thickness of from 200 to 1200 angstroms. 4.The window of any of claims 1, 2 or 3 wherein the optical element has asheet resistivity of less than 2.5 ohms per square.
 5. The window ofclaim 4 including flexible plastic layers within which said opticalelement is encapsulated.
 6. The window of claim 4 wherein thetransparent substrate for the optical element is glass.
 7. The window ofclaim 4 having a visible transmission of at least 70%.
 8. The window ofclaim 4 having a visible reflection of no more than 16%.
 9. The windowof claim 4 having a solar rejection of a least 50%.
 10. The window ofclaim 5 wherein the flexible plastic comprises polyvinyl butyral.
 11. Atransparent, laminated window capable of transmitting at least 70% ofvisible light, rejecting at least 50% of solar radiation and havingimproved defrosting capability, said window comprising:(A) an opticalelement having a sheet resistivity less than 3 ohms per squarecontaining:(i) at least two electrically conductive metal layers; (ii)at least one dielectric spacing layer between said conductive layers;and (iii) a transparent substrate; and (B) a bus bar electricallyconductively associated with each conductive layer.